I voted constitutional crisis and erosion of political institutions & the public's faith in them. Although environmental damage is also a very serious one. The silver lining on that one is we're far enough along the move towards renewable energy that there's no chance of coal reviving, and renewables (and in the short term, natural gas) will continue to gain. But there's still a lot of additional pollution and reversed environmental regulations that could kill large numbers of people and/or contribute significantly to global warming. I have no faith in the EPA under Scott Pruitt.
Erosion of political institutions and the public faith seems almost inevitable. Besides being the only president with no relevant prior experience, and losing the popular vote by the second-largest margin ever (to John Quincy Adams), the ability of the executive branch to, err, execute effectively seems doubtful given its record so far, and his policies are only increasing polarization in the country, which will further erode public faith in government. The possible upside is a significant increase in political activism among liberals and independents, which could lead to longer-term civic engagement, assuming it doesn't lead to a yo-yo effect of alternating between populists and liberals and continually increasing polarization.
There are several ways I could see a constitutional crisis emerging, many of them exacerbated by questions about the president's willingness to cede power when the time comes for that. One of the big questions is ties to Russia and the possibility of treason, which could certainly provoke a constitutional crisis if the ties prove to be substantial, particularly if the president doesn't have the grace of Nixon in resigning. Even if the ties shown are more tenuous, it would significantly weaken public faith in the government and the executive branch's ability to advocate for policies. I would be surprised if at least some ties are not shown given the current evidence and the administration's denials, smear campaigns, and attacks on the media.
There also could be constitutional crises around civil rights and the Bill of Rights - already the Muslim Bans have rubbed up against that territory. Election results in 2020 could also provoke one if they are close and there are allegations of the election being rigged (or even in 2018, although that seems less likely). I also have misgivings about the possibility (likely at this point) of Senate Republicans being willing to "go nuclear", not so much with the filibuster itself, but potentially in other areas in the future. Their refusal to so much as hold a confirmation hearing for Merrick Garland in 2016 has already put them at the lowest possible rung of respect for normal government procedures in my opinion. And while this may not be directly attributable to the president, I doubt they would have tried to delay another four years had Clinton won the electoral college as well as the popular vote.
There is the possibility of war as well, but I see significantly damaging America from within as a considerably more likely scenario.
